Aqueous suspensions of ceramic particles, such as magnesium, calcium and yttrium oxide, are used industrially to form ceramic articles. A suspension is a system in which small particles, typically solid particles, are uniformly dispersed in a liquid, such as water. Particles on the order of less than about 5 .mu.m may be classified as colloidal particles, and a suspension of such particles is referred to as a colloidal suspension. Ceramic suspensions are used to make ceramic objects. A ceramic is a hard, brittle, heat- and corrosion-resistant material that may be produced by forming the ceramic particles into a desired shape and then firing the shape to its final density. Ceramic suspensions used to make ceramics typically include more than one type of particle, and also may include binders and surfactants.
Ceramics normally are at least partially soluble in water. Furthermore, ceramics may hydrate, that is the ceramics may react with water to form a bond. The compounds that result from the hydration are referred to as hydrates. To what extent and how quickly ceramics dissolve (the dissolution rate) or hydrate varies. Moreover, colloidal particles of ceramics may agglomerate in water. The extent to which ceramics dissolve, hydrate or agglomerate in water depends on many factors, including the nature of the ceramic, the oxidation state of the ceramic, the pH of the system and temperature. Unagglomerated colloidal suspensions are useful for many ceramic applications. Persons skilled in the art of ceramic processing have long sought methods to stabilize colloidal suspensions, i.e., preventing the suspensions from agglomerating, while simultaneously reducing the dissolution and hydration rates.
Aqueous colloidal suspensions can be dispersed, and such dispersion may proceed by a number of mechanisms. For instance, three known mechanisms include electrostatic, steric, and electrosteric mechanisms. These mechanisms are reviewed by Cesarano and Aksay, "Stability of Aqueous .alpha.-Al.sub.2 O.sub.3 Suspensions with Poly(methacrylic acid) Polyelectrolyte," J. Am. Ceram. Soc., 71:250-255 (1988) . Electrostatic stabilization involves creating like charges on the surface of colloidal particles so that such particles repel each other, thereby dispersing the suspension of such particles. Id. Steric stabilization involves adsorbing relatively-large polymeric compounds onto the surface of particles. Parts of the polymer become strongly attached to the surface of particle, whereas the rest of the polymer may trail freely in the aqueous medium. If the aqueous medium is a good solvent for the polymer, interpenetration of polymer chains, i.e., the interaction of polymers on separate particles, is not energetically favorable. As a result, individual particles repel each other (interparticle repulsion), thereby dispersing the suspension. Id.
Finally, electrosteric stabilization is a combination of electrostatic and steric stabilization. Electrosteric stabilization involves adsorbing charged polymers (polyelectrolytes) on the surface of colloidal particles. The surface of a ceramic particle normally is composed of negative as well as positive sites. For instance, such sites might include functional groups such as .sup.- OH, H.sup.+, O.sup.2-, O.sup.-, etc. The relative concentration of each charge depends on a number of factors such as the nature of metal, the oxidation state of the metal, and the pH of the system. Polyelectrolytes have associated with them an overall electrical character (i.e., positive or negative). Polyelectrolytes adsorb strongly to the surface of particles by attaching themselves to oppositely charged sites on the surface of particles. However, not all the ionic sites on each polyelectrolyte are used during the adsorption process. Some of the ionic sites will be used to adsorb the polyelectrolyte to the surface of the ceramic particle, whereas some of the ionic sites will be in the part of the polymer that trails freely in the aqueous medium. The combined like charges associated with the particle surface and polymer chains in solution give each particle an overall negative or positive charge for the particle-polymer composition. Each "polymer-coated" particle will repel the like charges associated with a second "polymer-coated" particle because such particles experience an electronic repulsion. This electronic repulsion, in combination with the steric effect of the polymer, disperses the suspension. Id.
Organic polymers may also be used as particle growth inhibitors during solution precipitation methods of particle formation. Formation of unagglomerated particles with controlled size is highly desirable. As solid particles nucleate from ions in solution, uncontrolled growth and agglomeration may occur. If initial precipitating salt solutions contain dispersing polymers, available dispersing agents in solution adsorb on the surface of growing particles and inhibit their further growth and agglomeration.
Suitable organic polymers that may be used for stabilizing ceramics typically have "anchoring sites." Anchoring sites are chemical functional groups, particularly groups that ionize in aqueous suspensions or solutions. The anchoring sites are attracted to the particle. Polymeric compounds having many types of anchoring sites are suitable for adsorption onto ceramic surfaces. Such functional groups include, for example, a carboxyl (-COOH), a sulphonic acid (-SO.sub.3 H) and a phosphoric acid (PO.sub.3 H). These functional groups are suitable for adsorption into positive sites associated with the ceramic surface. Amines (R.sub.2 NH) and quaternary ammonium salts (R.sub.4 N.sup.+) are examples of functional groups suitable for adsorption into negative sites associated with ceramic surfaces.
Besides colloidal dispersion, reducing the attack by water (i.e., hydration and/or solvation) on the ceramic particle also is an important consideration for making commercially suitable ceramic slurries. Ceramic materials normally react with water and either partially dissolve (referred to as dissolution or solvation) or form hydrates. The extent of dissolution or hydration varies among different ceramic materials. As ceramic materials dissolve, the dissolved species may substantially change the ionic strength of the solution and consequently agglomerate the particles. Furthermore, if the charge of the species that dissolves is different from the charge of the ceramic particles or other components of the slurry (e.g., the ceramic particles have negative charge, but the dissolving species have positive charge or vice versa), then the dissolving species may preferentially adsorb onto different components of the slurry. This may change the rheological (the deformation and flow of matter) properties of the slurry, as well as other properties of interest in the slurry. Finally, as ceramic materials react with water, some ions may preferentially dissolve and consequently change the ratio of ions in each particle. This may result in changes in the physical or chemical properties of the ceramic.
In the case of particle hydration, some ceramics form an hydroxide surface layer. However, attack by water also may proceed farther than the surface layer and may advance into the body of the particle. As a result, size, morphology, and the crystal phase of the particles may change. In many commercially important ceramics, such as alumina (Al.sub.2 O.sub.3), zirconia (ZrO.sub.2), and zircon (ZrSO.sub.4), to name a few, the dissolution rate and the extent to which dissolution proceeds is low enough so that it does not seem to interfere with their aqueous commercial use, at least under mild acidic or basic conditions, such as from about pH 3 to about pH 11. Furthermore, hydration does not seem to form more than a thin surface layer, at least when the particle size is equal to or larger than one micrometer.
However, other commercially important ceramics, such as magnesia (MgO), calcia (CaO) and yttria (Y.sub.2 O.sub.3), to name a few, dissolve in an aqueous media to a much larger extent and at faster rates than the ceramic materials discussed above. In some other commercially important ceramics, such as high T.sub.c superconductors or perovskites, one cation may preferentially dissolve in water and alter the cationic ratio of the initial powder. Perovskite typically is a natural or synthetic crystalline mineral composed of certain oxide compounds, particularly titanates. Examples of Perovskites, without limitation, include lead titanate (PbTiO.sub.3), barium titanate (BaTiO.sub.3) and strontium titanate (SrTiO.sub.3). As a result, aqueous processing of these material, such as magnesia, calcia, yttria, perovskites, and high T.sub.c superconductors, is either difficult or impractical under most conditions. Furthermore, MgO, CaO, and aluminum nitrides (AlN), to name a few, are known to be attacked by water and hydrate extensively, such as deeper than just a surface layer.
Many attempts have been made by persons skilled in the art of ceramic processing to reduce the dissolution and hydration of ceramic particles, while simultaneously keeping the ceramic particles dispersed (unagglomerated) in suspensions. Of the three methods for stabilizing colloidal suspensions mentioned earlier, electrostatic is the least favorable method for reducing water attack. Electrostatic stabilization is normally achieved by adjusting the pH of the suspension to a pH suitable to produce a particle surface charge that is high enough to induce dispersion. When this method of colloidal stabilization is utilized, the particle surface is in direct contact with the aqueous environment. As a result, the aqueous medium attacks the surface of the ceramic particle, and the particles either dissolve or hydrate.
In limited cases, however, water attack can be reduced by electrostatic stabilization. For example, Horton's U.S. Pat. No. 4,947,927 teaches that by adjusting the pH of a yttria slurry to high pH values in excess of pH 11 one can make yttria intrinsically less soluble in water, thereby decreasing its sensitivity to water attack. This approach, however, has great limitations and is not generally applicable to other materials.
Compared to electrostatic stabilization, electrosteric stabilization apparently provides a better method for simultaneously dispersing colloidal particles in suspension and reducing water attack on the ceramic surface. As polyelectrolytes strongly adsorb onto a ceramic surface, the polyelectrolyte provides a barrier layer between water molecules and the ceramic surface, thereby reducing the effect of water on the ceramic particle. This method initially provides a shield against water attack. However, it has been shown that if the dissolving ions have opposite charges compared to the anchoring sites of the polyelectrolyte, as the ions dissolve into solution, they bond with oppositely charged anchoring sites on the polyelectrolytes (anchoring sites which are not adsorbed on the surface), thereby either (1) precipitating the polyelectrolyte out of solution, or (2) forming a bridge between different particles and causing them to agglomerate. H. Nakagawa, M. Yasrebi, J. Liu, and I. A. Aksay, "Stability and Aging of Aqueous MgO Suspensions," presented at the annual meeting of the Am. Ceram. Soc. (1989) The limitations of polymethacrylic acid and the stabilization of MgO are demonstrated below in Example 2.
Steric stabilization is an even more attractive method for protecting ceramic surfaces from water attack compared to electrostatic or electrosteric stabilization. Stabilizing polymers, which do not include charged sites in their unabsorbed tails and loops, provide a barrier between the surface and the water molecules; hence, stabilizing polymers do not interact with dissolving species as in the case of polyelectrolytes.
Although the above discussion is generally true, steric stabilization also has its own practical limitations. For instance, as the ratio between the number of hydroxyl groups relative to acrylic-acid groups increases for an acrylic acid-vinyl alcohol copolymer, the copolymer becomes a more effective barrier for protecting MgO from attack by water in aqueous environment. See JP 81 73 623, and JP 81 73 624. However, as this ratio increases, the polymer loses its effectiveness as a shield between the surface of the particle and the water molecules. Although no explanation is offered in the above patents for this observed behavior, apparently as the ratio between the number of hydroxyl and acrylic acid groups increases, the affinity of the polymer for the ceramic surface is reduced to a point where no substantial surface adsorption occurs. Thereafter, the copolymer becomes ineffective.
Another common method for protecting ceramic particles from attack by water has been to coat the particles with a thin layer of hydrophobic material. Common materials for this purpose include different waxes. Although this method keeps water away from the particle surface, the particles become more hydrophobic as the amount of hydrophobic material used to coat the particle increases. As the hydrophobic nature of the particles increase, the particles agglomerate, and thus must be redispersed in water by adding dispersing agents such as surfactants. The need for additional dispersing agents makes this method less practical. More importantly, due to either insufficient uniformity of wax coating, or rapid removal of wax from the surface, this method has not proven to be effective.
Still other methods of reducing water attack have been used. Surface coating or fusion of particles with an inorganic material that is more inert to water attack has proven somewhat effective in reducing the water sensitivity of some ceramics. However, the foreign inorganic material is incorporated into the ceramic end product, which is unacceptable in most situations.
Recently, monomers have been used to prevent the agglomeration of alumina suspensions. Graule et al., "Stabilization of Alumina Dispersions with Carboxylic Acids," Proceedings of the Second European Ceramic Society Conference (1991). Graule discusses the ability of certain carboxylic acids to stabilize high-purity alumina suspensions in aqueous media. More specifically, Graule studied dispersing .alpha.-Al.sub.2 O.sub.3 powder using L-lactic acid, DL-malic acid, tartaric acid, tricarballylic acid, citric acid, 2-hydroxybenzoic acid, 3-hydroxybenzoic acid, and 4-hydroxybenzoic acid. Graule teaches that the efficiency of hydroxy carboxylic acids to act as dispersion agents for .alpha.-Al.sub.2 O.sub.3 increases over the series from lactic, malic, tartaric, tricarballylic to citric acid. Moreover, the efficiency of hydroxybenzoic acids increases from 4- to 3- to 2-hydroxybenzoic acid.
More recently, T. Ren, I. A. Aksay, M. Yasrebi, N. B. Pellerin, and J. T. Staley, have investigated Graule's work in more detail. T. Ren et al., "Hydroxylated Carboxylic Acid Monomers Serve as Dispersants for Ceramic Particles," (unpublished works). T. Ren et al. used twenty-one carboxylic acids to evaluate the ability of carboxylic acid. The acids tested had from one to four carboxyl (--CO.sub.2 H) and/or carboxylate groups (CO.sub.2.sup.-), depending on the pH of the suspension. Table I below shows the formulas of the acids tested.
The dispersion tests with the acids shown in Table I have shown that some of the carboxylic acids are better dispersants for colloidal suspensions of alumina. In summary, in confirmation of Graule's work, T. Ren et al. have observed that hydroxyl groups enhance the ability of the monomer to disperse the colloidal metal-oxide particles. Moreover, they have concluded that for compounds that contain the same number of carboxyl groups and which have a similar structure, it appears that as the number of hydroxyl groups increases, the better the carboxylic acid functions as a dispersant.
TABLE I ______________________________________ Carboxylic Acids Investigated in Ren et al.'s Study With Their Molecular Components Name Structure ______________________________________ A. Mono-carboxylic acids 1. Glutamic --OOCCH(NH.sub.3.sup.+) (CH.sub.2).sub.2 COOH 2. Mannuronic HOOC[CH(OH)].sub.4 CHO B. Di-carboxylic acids 1. Adipic HOOC(CH.sub.2).sub.4 COOH 2. Mucic HOOC[CH(OH)].sub.4 COOH 3. Fumaric (trans) HOOCCH.dbd.CHCOOH 4. meso-Tartaric HOOCCH(OH)CH(OH)COOH 5. Succinic HOOC(CH.sub.2).sub.2 COOH 6. Maleic (cis) HOOCCH.dbd.CHCOOH 7. Malic HOOCCH.sub.2 CH(OH)COOH 8. d,l-Tartaric HOOC[CH(OH)].sub.2 COOH 9. Malonic HOOCCH.sub.2 COOH 10. Tartronic HOOCCH(OH)COOH 11. Mesoxalic HOOCCOCOOH 12. Glutaric HOOC(CH.sub.2).sub.3 COOH 13. Sebacic HOOC(CH.sub.2).sub.8 COOH C. Tri-carboxylic acids 1. Tricarballylic HOOCCH.sub.2 CH(COOH)CH.sub.2 COOH 2. Aconitic (trans) HOOCCH.sub.2 C(COOH).dbd.CHCOOH 3. Nitrilotriacetic HOOCCH.sub.2 N(CH.sub.2 COOH)CH.sub.2 COOH 4. Citric HOOCCH.sub.2 C(OH)(COOH)CH.sub.2 COOH D. Tetra-carboxylic acids 1. Ethylenediamine HOOCCH.sub.2 N(CH.sub.2 COOH)CH.sub.2 CH.sub.2 N tetracetate (EDTA) (CH.sub.2 --COOH)CH.sub.2 COOH ______________________________________
Graule and his co-workers speculate that the dispersion efficiency of hydroxy carboxylic acids depends on their ability to form chelate complexes. Furthermore, Ren and his co-workers have shown that apparently the hydroxyl group enhances the adsorption of the acid onto the alumina particles. In support of this possibility, they have found that, in adsorption tests, citric acid and meso-tartaric acid always absorb to a greater extent than do the structurally similar compounds, tricarballylic acid and fumaric acid. Citric acid and meso-tartaric acid have hydroxyl groups, whereas tricarballylic acid and fumaric acid do not. In addition, percent ionic dissociation of all four monomers were measured versus pH and was shown to be very similar. Ren and his co-workers concluded that the presence of a hydroxyl group may aid the adsorption of the acid to the colloidal particle. This increased adsorption also may explain the increased ability of such acids to disperse colloidal suspensions of alumina.
Graule et al. and Ren et al. address only dispersion of alumina suspensions. Their work suggests that certain monomers can be used as effective dispersing agents for alumina suspensions. As discussed above, alumina is a relatively stable metal-oxide in an aqueous environment. The limited reaction of alumina with water does not interfere with its aqueous processing in the pH range normally used for ceramic processes, i.e., about a pH of 3-11. Neither Graule nor Ren were concerned with, and apparently did not consider, the reaction of .alpha.-Al.sub.2 O.sub.3 with water.
The dissolution of iron oxide particles in the presence of aqueous solutions of ethylenediaminetetraacetic acid (EDTA), and its related aminocarboxyllic acids, has been studied by E. Matijevic and his co-workers. See, for example, R. Torres, M. A. Blesa, and E. Matijevic "Interaction of Metal Hydrous Oxides with Chelating Agents. XI. Reductive Dissolution of Hematite and Magnetite by Aminocarboxyllic Acids," J. Colloid and Interface Sci., Vol. 134, No. 2 (1990). Matijevic et al. discuss the fact that chelate-forming monomers normally are used to enhance particle dissolution. Matijevic et al. also demonstrated that in some cases the adsorptions of these chelating agents inhibited dissolution, whereas in other cases the chelating agents increased dissolution. In either case, however, Matijevic et al. showed that an interaction occurs between dissolving ions and adsorbed chelating agents. As discussed above, such an interaction eventually results in particle flocculation.
In summary, ceramic aqueous suspensions have a variety of industrial applications. Prior-art methods for stabilizing such suspensions rely primarily on polymeric or electrostatic stabilization. These methods often perform poorly for water-reactive ceramics. Graule's recent work as discussed above shows that hydroxylated carboxyl-group containing monomers may be used to disperse alumina, but such work teaches nothing about addressing the problems of particle dissolution and hydration using monomers. On the other hand, Matijevic et al.'s work on particle dissolution primarily emphasized enhancing the effects monomers have on particle dissolution. Hence, a need still exists for an improved method for stabilizing colloidal ceramic particles, while simultaneously reducing the rate of dissolution and/or hydration of such particles.